Combustion misfires lead to an increase of the toxic substances emitted during operation of the engine and can, in addition, lead to damage of the catalytic converter in the exhaust-gas system of the engine. A detection of combustion misfires in the entire rpm and load ranges is necessary to satisfy statutory requirements as to on-board monitoring of exhaust-gas relevant functions. In this context, it is known that, during operation with combustion misfires, characteristic changes occur in the rpm curve of the engine compared to normal operation without misfires. Normal operation without misfires and operation with misfires can be distinguished from a comparison of these rpm curves.
A detection system for combustion misfires includes the following function blocks: sensors, signal processing and feature extraction as well as classification (FIG. 1). The sensors detect, for example, segment times, that is, the time intervals in which the crankshaft passes through a predetermined rotational angle. Feature signals are formed from the segment times in the feature extraction block. The classification block follows the feature extraction block and combustion misfires are detected from the feature signals, for example, by threshold value comparisons or by utilizing a neural network or other known methods.
A system operating on the basis of threshold-value comparisons is already known and disclosed in German patent publication 4,138,765 which corresponds to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned.
In this known method, the segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time in which the crankshaft passes through this angle range is dependent, inter alia, upon the energy converted in the combustion stroke. Misfires lead to an increase of the ignition-synchronously detected segment times. Pursuant to the known method, a criterion for the rough-running of the engine is computed from the differences of the segment times. In addition, slow dynamic operations such as the increase of the engine rpm for vehicle acceleration are mathematically compensated.
A rough-running value, which is computed in this way for each ignition, is likewise compared ignition-synchronously to a predetermined threshold value. Exceeding this threshold value is evaluated as a misfire. The threshold value is dependent, as may be required, from operating parameters such as load and engine speed (rpm). This method is then based on the feature extraction in the time range.
Additional methods are known from U.S. Pat. Nos. 5,200,899 and 5,239,473 wherein the transformation of rpm signals in the frequency range by means of discrete fourier transformations is used for feature extraction. The display of the results indicates a block-like application of the transformation to the rpm signals. A block is formed of, for example, m rpm signals. The m rpm signals are determined during a camshaft revolution. However, individual misfires are not optimally resolved in this way. In contrast, if the transformation is performed sequentially, that is, if the evaluation block of m rpm signals is shifted by less than a camshaft rotation, then disadvantages result with respect to the detection of permanent misfires.
The above methods define the results of a block-like analysis in the frequency range and do not consider the further evaluation of the features obtained.
In contrast to the above, the invention includes a complete classification system for combustion misfires. The system is based upon a feature signal which is obtained by evaluating the rpm fluctuations utilizing signal modulation. Obtaining such a feature signal is disclosed in U.S. Pat. No. 5,691,469 which is incorporated herein by reference.
Briefly summarized, this patent application discloses a method for detecting combustion misfires which operates on the basis of a first signal in which the nonuniformity of the rotational movement of the crankshaft of the engine is mapped and wherein a second signal is generated which is periodical and which has a period duration which corresponds to the period duration of a work cycle of the engine or this period duration divided by a whole number k. The method also has a fixed phase relationship to the rotational movement of the crankshaft and the second signal is modulated by a signal based on the first signal and filtered. Also in the method, the modulated third signal, which results from the modulation and filtering, is used to detect combustion misfires and to identify the affected cylinder. The third signal defines an input feature signal with reference to the next-following classification stage wherein the nonuniformity of the rotational movement of the crankshaft of the engine is mapped in the k-th order.
The feature signal obtained in this way contains frequency data as well as time range data with which the signal-to-noise ratio significantly increases for the next-following classification stages.
By limiting to the first order (that is, k=1) for the feature extraction, then, for example, permanent misfires of cylinders, which are arranged symmetrically in the ignition sequence, cannot be detected.
Furthermore, several combinations of permanent misfires of several cylinders (multiple misfires) have components of the first order which lead to an incorrect detection as permanent misfires of a cylinder and to incorrect cylinder identification.